The present invention relates generally to high-speed data communication systems, and specifically to digital subscriber line systems.
Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire, which is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. DSL thus opens the most critical bottleneck in local-loop access to high-speed networks, such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) networks, without requiring major investments in new infrastructure. A range of DSL standards have been defined, known generically as xe2x80x9cxDSL,xe2x80x9d wherein the various standards have different data rates and other associated features but share common principles of operation.
DSL subscribers are connected to high-speed networks through Digital Subscriber Line Access Multiplexer (DSLAM) systems. Because of the high cost of network bandwidth, a single DSLAM must typically be designed to serve between 100 and 1000 subscribers and to concentrate their traffic through one or a few network trunks. The need to serve such a large and potentially variable number of subscribers in the one DSLAM has led to the development of xe2x80x9cmulti-shelfxe2x80x9d access architectures.
FIG. 1 is a block diagram that schematically illustrates a DSLAM system 20 implementing a multi-shelf architecture, as is known in the art. System 20 comprises a master unit 25 coupled to multiple slave units 27 in a star configuration. Master unit 25 communicates with a core network 22, such as an ATM network. The master unit comprises a core network interface element 24, providing the necessary physical layer (PHY) and data link layer (for example, ATM) functions. A concentrator 26 performs higher-level functions, including VPI/VCI translation for the ATM network, and multiplexes downstream and upstream packets, or cells, among slave units 27. Each slave unit typically comprises a switching core 29, coupled to a plurality of ports 28 serving respective subscriber premises via suitable DSL modems. In the physical implementation of system 20, each such slave unit occupies its own shelf in an electronics rack, and this is the reason for the use of the term xe2x80x9cmulti-shelf.xe2x80x9d
FIGS. 2A and 2B are block diagrams that schematically illustrate topologies known in the art for use in multi-shelf access systems, such as that shown in FIG. 1. FIG. 2A shows a simple star configuration. In a redundant star configuration, shown in FIG. 2B, each slave unit is connected to two or more alternative master units, so as to provide protection in case of a failure in one of the master units.
It is an object of some aspects of the present invention to provide an improved multi-shelf access architecture.
It is a further object of some aspects of the present invention to provide apparatus and methods for efficient, high-speed transfer of data packets within an access multiplexer system.
It is still a further object of some aspects of the present invention to improve the robustness and reliability of multi-shelf access systems.
In preferred embodiments of the present invention, a multi-shelf access system comprises two master units (xe2x80x9cmastersxe2x80x9d) and a plurality of slave units (xe2x80x9cslavesxe2x80x9d) One of the masters serves as an active master, through which the slaves normally receive data packets from a core network and transmit packets back upstream to the network. The other master is termed a standby master. The slaves are connected in one or more daisy chains between the active and standby masters, and are configured so that both downstream and upstream packets can be transmitted in either direction along each of the chains, i.e., from or to either of the two masters. Thus, if a failure occurs in any one of the slaves or in a link between them, the traffic direction in the chain in which the failure has occurred is simply reversed so as to run through the standby master. Substantially no reprogramming of the slaves is required for this purpose. All of the slaves (except possibly a slave that has failed) continue to provide service to subscribers, typically with only a short interruption until corrective action is completed.
Preferably, the upstream packets are bicast by the active master to both the core network and to the standby master. The standby master also transmits the upstream packets to the core network, in order to protect against faults in the network interface of the active master unit.
In some preferred embodiments of the present invention, each slave comprises a pre-switch, which examines each packet passed down its respective chain to determine whether the packet is destined for any of the ports on that slave. Preferably, every packet passed between the masters and slaves has a pre-switch address. If the packet is addressed to a particular slave, the pre-switch on that slave conveys the packet to a switch or switching fabric of the slave, which routes the packet to the appropriate port. If the packet is a multicast packet with an appropriate pre-switch address, the pre-switch both conveys a copy of the packet to the switch or switching fabric of the slave and passes the packet on down the chain. The pre-switch similarly accepts packets carrying a xe2x80x9cnextxe2x80x9d address, i.e., packets that are sent from one slave to its immediate neighbor for topology detection and diagnostics. Otherwise, the pre-switch rejects the packet and passes it on to the next slave along the chain.
The pre-switch thus reduces the burden on the switching fabric of the slave and ensures that packets are passed down the chain at the full data rate supported by the master-slave interface. Furthermore, in the case of a fault in the slave, the pre-switch simply passes all packets through to the next slave in the chain, so that the effect of the fault is limited as far as possible.
Preferably, the slave pre-switch is bidirectional, i.e., it treats packets passed both up and down the chain in substantially the same manner. The pre-switch thus supports the failure protection mechanism described hereinabove, wherein the traffic direction is reversed in response to a failure in the chain.
There is therefore provided, in accordance with a preferred embodiment of the present invention, network access apparatus, including:
first and second master units, each including a physical interface to a packet-switched network;
a plurality of slave units, each slave unit including one or more ports to respective subscriber lines; and
a plurality of physical interface lines, which link the slave units in one or more daisy chains, in which the slave units are mutually connected in series by the physical interface lines therebetween, each daisy chain including at least a first slave unit connected by one of the physical interface lines to the first master unit and a last slave unit connected by another of the physical interface lines to the second master unit.
Preferably, in normal operation, downstream data packets received from the network are passed from the first master unit to each of the daisy chains via the first slave unit in each chain, and upstream data packets received by the slaves in each chain from the subscriber lines are passed via the first slave unit in the chain to the first master unit for transmission over the network. Further preferably, the apparatus includes a protection interface, which couples the second master unit to the first master unit, and over which interface data packets are conveyed between the first and second master units in case of a fault. Most preferably, the first master unit bicasts the upstream data packets that it receives from the slave units to the network and, via the protection interface, to the second master unit, which transmits the upstream data packets to the network.
Preferably, in case of a fault at a location in one of the daisy chains, data flow in a portion of the daisy chain between the location of the fault and the second master unit is reversed, so that the downstream data packets are passed from the second master unit to the slave units in the portion of the daisy chain via the last slave unit in the chain, and the upstream data packets are passed by the last slave unit to the second master unit. Further preferably, the downstream packets for the slave units in the portion of the daisy chain between the location of the fault and the second master unit are conveyed to the second master unit from the first master unit via another one of the daisy chains.
In a preferred embodiment, each of the first and second master units includes:
a switch, configured to route data packets between the respective physical interface and the one or more daisy chains; and
a pre-switch, which in the event of a fault at a location in one of the daisy chains, re-routes at least a portion of the data packets exchanged with one or more of the slaves in the daisy chain in which the fault has occurred through another one of the daisy chains. Preferably, the pre-switch re-routes the data packets such that substantially no reconfiguration of the switch is required responsive to the fault.
In a further preferred embodiment, each of the slave units includes:
a switch fabric including one or more switches, which convey data packets to respective ports on the switch to which the packets are addressed; and
a pre-switch, which receives the data packets from one of the physical interface lines connected to the slave unit and passes those of the data packets that are addressed to any of the ports on the slave unit to the switch fabric, while passing packets not addressed to any of the ports on the slave unit for output through another of the physical interface lines.
Preferably, each of the slave units is coupled to receive packets transferred thereto from the first and second master units over first and second ones of the physical interface lines, respectively, and the pre-switch passes the packets received through the first and second physical interface line and addressed to any of the ports on the slave unit to respective first and second addresses in the switch fabric. Further preferably, in response to a reversal of a direction of data flow in the daisy chain, the first and second addresses are swapped in the pre-switch, so that substantially no reconfiguration of the switch fabric is required.
In a preferred embodiment, the network includes an asynchronous transfer mode (ATM) network. In another preferred embodiment, the network includes an Internet protocol (IP) network.
There is further provided, in accordance with a preferred embodiment of the present invention, in a network access multiplexing system, in which a master unit is connected by a physical interface to a packet-switched network, a slave unit configured to be coupled to the master unit in a daisy chain of such slave units, the slave unit including:
a plurality of ports, for coupling the slave unit to respective subscriber lines;
first and second physical interfaces, coupled to exchange packets with preceding and succeeding units, respectively, along the daisy chain;
a pre-switch, coupled to receive packets from the first physical interface and responsive to address data carried by the packets, to sort the packets such that packets addressed to the slave unit are retained, and packets addressed to the succeeding units are passed to the second physical interface; and
a fabric of one or more switches, which convey the retained packets to the ports, responsive to the address data.
Preferably, the pre-switch is further coupled to receive packets transferred thereto from the second physical interface and to sort the packets in like manner to the packets received through the first physical interface. Most preferably, the retained packets that were received from the first and second physical interfaces and are passed by the pre-switch to the switch fabric are identified by respective first and second port numbers, wherein in response to a reversal of a direction of data flow in the daisy chain, the first and second port numbers are swapped in the pre-switch, so that substantially no reconfiguration of the switch fabric is required in response to the reversal.
Further preferably, when one of the packets received by the pre-switch includes a multicast packet addressed to one or more of the ports on the slave unit, the pre-switch sorts the multicast packet such that one copy of the packet is retained and another copy of the packet is passed to the second physical interface.
Preferably, in the event of a fault in the switch fabric, the pre-switch continues to pass the packets addressed to the succeeding units on to the succeeding units without significant interruption.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for providing access to a network, including:
coupling first and second master units to interface with the network;
linking a plurality of slave units, each slave unit including one or more ports to respective subscriber lines, in a daisy chain between the first and second master units;
conveying initial downstream data packets, received from the network by one of the master units, along the daisy chain in a first direction, so as to deliver the packets to the ports of the slave units; and
in the event of a fault in the daisy chain, conveying further downstream data packets, received from the network by one of the master units, along the daisy chain in a second direction, opposite to the first direction, so as to deliver the further packets to the ports of at least some of the slave units.
Preferably, the initial and further downstream packets are received from the network by the first master unit, and conveying the further downstream packets in the second direction includes conveying the further downstream packets from the first master unit to the second master unit, and then conveying the further downstream packets from the second master unit to the daisy chain. Most preferably, conveying the further downstream packets from the first master unit to the second master unit includes linking further slave units in an additional daisy chain between the first and second master units, and conveying the further downstream packets from the first master unit to the second master unit over the additional daisy chain.
Further preferably, the method includes conveying initial upstream data packets, received by the slave units from the subscriber lines, along the daisy chain in the second direction so as to transmit the upstream data packets via the first master unit over the network, and in the event of the fault, conveying further upstream data packets received by one or more of the slave units along the daisy chain in the first direction via the second master unit. Most preferably, the method includes bicasting the upstream data packets from the first master unit to the network and to the second master unit, which transmits the bicast upstream data packets over the network.
Preferably, conveying the initial downstream data packets along the daisy chain includes pre-switching the packets at each of the slave units, so that packets not addressed to any of the ports on the slave unit are passed to the next slave unit in the daisy chain, while packets that are addressed to one or more of the ports on the slave unit are passed to a switch fabric that directs the packets to the ports to which they are addressed.